This invention relates to an apparatus for navigating a vehicle and, more particularly, to a vehicle navigation system for guiding a traveling vehicle based on assuming the present position of the vehicle from the direction in which the vehicle is traveling and from the distance traveled from a reference point, and effecting matching between the assumed present position and previously stored road data relative to a map.
Development of a type of vehicle navigation system based on dead reckoning has been progressing. In this type of vehicle navigation system, the present position of a vehicle is assumed from the compass direction in which the vehicle is traveling obtained by an azimuth sensor utilizing earth magnetism and from the distance traveled from a reference point obtained by a distance sensor, and the assumed present position and map data on the vicinity are displayed on a screen of a display device, thereby enabling guidance for the vehicle. Since this dead reckoning navigation system is designed to assume the present position of the vehicle relative to the reference point on the basis of the direction obtained by the azimuth sensor and the distance traveled obtained by the distance sensor, errors in measurement of the distance traveled and the direction are accumulated as the vehicle moves away from the reference point, resulting in a reduction in the accuracy with which the present position can be determined.
To eliminate the defects of this dead reckoning navigation system, a technique called "map matching navigation" has been developed. An example of map matching navigation is disclosed in Japanese Patent Laid-Open No.61-209316 in which the arrangement is such that when the vehicle passes over a point where the road has a specific feature such as an intersection, the present position of the vehicle assumed previously by dead reckoning is corrected to the position of that distinctive point. If the number of distinctive points where map matching is to be performed when the vehicle passes is large enough to ensure that each distinctive point is reached before the accumulated error due to dead reckoning becomes excessive, map matching can be performed correctly at the destined distinctive points. It is therefore possible to say that map matching navigation is free from the drawbacks of the conventional dead reckoning navigation.
In this known map matching navigation, a judgment is made to determine which one of a plurality of distinctive points is being passed by the vehicle in such a manner that the distinctive point which is closest to the present position obtained by dead reckoning is simply regarded as the place which the vehicle is currently passing.
However, in the case of matching a road on the map with the present position of the vehicle, as assumed with a certain measurement error, there is a possibility of failure to properly match the vehicle position with the road on which the vehicle is actually traveling. To understand this possibility, the following case may be assumed. If there are a plurality of roads extending relatively close to each other, and if the vehicle travels along one of these roads, it is possible that two or more intersections simultaneously appear to be close to the vehicle. In such a case, it is difficult to effect correct matching, and the present position of the vehicle may be matched with a road that is not actually that along which the vehicle is traveling. Once the present position of the vehicle has been matched with a wrong road, it is impossible to correctly perform subsequent matching operations.
In particular, in urban regions, many roads intersect each other at closely spaced locations. If map matching navigation is conducted in such a region, the possibility of failure to effect correct matching is high. Conventionally, no means for detecting passing and traveling along a corner has been used, and the accuracy in map matching navigation for traveling in urban regions has therefore been inadequate.
A method for map matching in which all roads are previously memorized and the vehicle position obtained by dead reckoning is constantly matched with corresponding points on the memorized roads is also known. However, there are roads which are not represented by any map information for various reasons. For example, in the case of a newly constructed road or in the case of a narrow road with little traffic, the possibility of information on the road being omitted is high. Map matching for traveling on a road which is not represented on the map necessarily results in failure to match the present position with the actual road.
An azimuth sensor for detecting the direction in which the vehicle travels is indispensable for dead reckoning, and the operation of the azimuth sensor entails a degree of error. For example, if the azimuth sensor is an earth magnetism sensor, measurement data output from the sensor contains an error due to magnetization. If such an error is ignored, the amount of accumulated error becomes large, resulting in a reduction in the map matching accuracy.